Loft Conversion Types Compared: Costs, Space & Planning in London (2026)
In London in 2026, a loft conversion costs from around £25,000 for a rooflight (Velux) conversion to £120,000+ for a mansard, with dormer and hip-to-gable conversions in between. But the right type is not chosen on price alone — it is decided by your roof: its shape, its head height, and what planning allows on your street. This guide compares the four main types side by side — rooflight, dormer, hip-to-gable and mansard — so you can see what each costs, how much usable space it actually adds, and when you need planning permission.
What are the four main types of loft conversion?
Nearly every London loft conversion is one of these four (or a combination):
- Rooflight (Velux) conversion — the simplest. You keep the existing roof shape exactly and set windows into the slope, then insulate, floor and fit out. No change to the roofline. Only works if you already have the head height.
- Dormer conversion — a flat-roofed box built out from the rear roof slope to create full-height headroom and floor area. The most common London loft conversion, and usually the best value for the space gained.
- Hip-to-gable conversion — a sloping "hipped" side roof is rebuilt vertically to a flat "gable" end, reclaiming the volume at the side. Only relevant to houses with a hipped roof — typically semi-detached, detached and end-of-terrace homes. Nearly always combined with a rear dormer.
- Mansard conversion — the biggest and most expensive. One or both roof slopes are rebuilt to near-vertical (around 70°) with a shallow top, adding most of a full storey. The go-to for London terraces that want maximum space and a roof form that suits a conservation area.
How much does each type of loft conversion cost in London in 2026?
The type dictates the price far more than the floor area, because each type does a different amount of structural work to the roof. These are realistic 2026 London figures, fully fitted — structure, staircase, insulation, electrics, plastering and decoration included:
| Loft type | Roofline change | Typical London cost (2026, fully fitted) | Space gained | Planning |
|---|---|---|---|---|
| Rooflight / Velux | None | £25,000 – £45,000 | Least — limited by the existing roof | Almost always permitted development |
| Dormer (rear) | Box out the rear slope | £45,000 – £75,000 | Good — full head height in the box | Usually permitted development |
| Hip-to-gable (+ rear dormer) | Fill the hip to a vertical gable | £55,000 – £85,000 | Large — best for semis & end-terraces | Often permitted development — check |
| Mansard | Rebuild the roof slope near-vertical | £60,000 – £120,000+ | Most — close to a full extra floor | Almost always needs planning permission |
Indicative 2026 London figures for a fully-fitted conversion. London runs roughly 20–30% above the national average on labour and access. Most loft firms quote a turnkey price — confirm whether VAT (standard-rated at 20% on this work) is included, and whether the staircase, decoration and any en-suite are in or out.
The shell is rarely the whole story. A rear dormer with a new en-suite, bespoke joinery and higher-spec glazing can cost more than a simple mansard shell; a hip-to-gable is priced up by the extra scaffolding and the rebuild of the side roof. Where two figures matter most: the amount of roof you rebuild and the fit-out spec inside.
Which loft conversion suits your house?
Your roof usually makes the decision for you:
- Victorian / Edwardian terrace — a rear dormer is the standard, best-value choice; a mansard where you want maximum space or where the conservation area favours that form.
- Semi-detached, end-of-terrace or 1930s house with a hipped roof — a hip-to-gable plus rear dormer unlocks the most space, because you reclaim the side hip as well as the rear.
- Detached house or bungalow — usually a hip-to-gable, often to both sides, with the largest 50 m³ permitted-development allowance to play with.
- Property that already has generous roof height — a rooflight (Velux) conversion is the cheapest and least disruptive, with no roofline change and often no planning needed.
The one hard test is head height. Measure from the finished floor to the underside of the ridge: you need roughly 2.2 m over at least half the floor area for a comfortable, building-regs-compliant conversion. Below about 2.2 m at the ridge and a standard rooflight or dormer conversion becomes marginal — which is exactly when a mansard (rebuilding the roof higher) or lowering the ceiling below starts to make sense.
Do you need planning permission for a loft conversion?
Many London loft conversions are permitted development (PD) and need no planning application — but the type and your location decide it. The core PD limits (Planning Portal, GPDO Class B) are:
- Volume allowance: up to 40 m³ of additional roof space for a terraced house, and 50 m³ for a semi-detached or detached house — including any previous roof additions.
- No extension beyond the existing roof slope on the elevation fronting a highway — so front-facing dormers almost always need full planning; rear dormers usually do not.
- No part higher than the existing ridge, materials similar in appearance, and no balconies or raised platforms (a Juliet balustrade inside the opening is usually fine).
- Set-back rule: side-facing dormers and roof extensions must be set back at least 20 cm from the eaves.
By type, in practice: a rooflight conversion is almost always PD; a rear dormer is usually PD within the volume limits; a hip-to-gable is often PD but the roofline change means you should confirm it; and a mansard almost always needs full planning permission because it re-shapes the roof.
PD rights fall away in the situations that cover much of inner London: flats and maisonettes have no PD rights, listed buildings need listed building consent, and in conservation areas and under Article 4 directions (common across Islington, Hackney, Camden, Kensington & Chelsea and others) rights are removed or reduced — so most period homes in those boroughs need a planning application. Where PD does apply, it is worth securing a Lawful Development Certificate to put the status beyond doubt before you build. See our London planning permission guide and our planning application service.
What building regulations apply to a loft conversion?
Every loft conversion needs building regulations approval regardless of planning, and a loft turns a two-storey house into a three-storey one — which triggers stricter fire rules:
- Fire safety (Approved Document B): the staircase must be enclosed in a protected stairway with 30-minute fire resistance all the way to a final exit, fire doors to habitable rooms off it, mains-powered interlinked smoke alarms, and an escape (egress) window in the loft sized for an adult to climb through.
- Head height: the practical working minimum is about 2.2 m over 50% of the floor, with reduced allowances over the new stair.
- Stairs (Approved Document K): a fixed staircase with at least 2 m headroom (reduced to ~1.9 m at the centre / 1.8 m at the edge for a loft stair).
- Structure: new steel beams (RSJs) to carry the roof and floor, sized by a structural engineer and signed off by building control.
- Insulation (Part L) to current standards, plus sound and ventilation requirements.
On a terrace or semi you will also almost certainly need party wall awards with neighbours either side — around £1,000–£2,500 per neighbour, with notice served roughly two months before work starts. See our party wall agreements service.
Which type adds the most space and value?
Mansard adds the most floor area — close to a full extra storey — followed by hip-to-gable plus dormer, then a rear dormer, with a rooflight adding the least. A loft that gains a genuine double bedroom with an en-suite is consistently one of the strongest returns on a London home, often adding more to the sale price than it costs to build.
The constraint is your street's ceiling price — the realistic maximum even a beautiful finish will not beat. If you are already near it, a rooflight or modest dormer protects your margin; if you are well below it, the extra spend on a mansard or hip-to-gable usually pays back. Get a local agent's view on your ceiling price before you fix the loft type. If you are weighing the loft against building out instead, read our loft conversion vs rear extension comparison.
How long does a loft conversion take?
Design, drawings, planning or a Lawful Development Certificate, and party wall typically take 2–3 months before you start on site. On site: a rooflight conversion is roughly 4–6 weeks, a dormer 6–8 weeks, and a hip-to-gable or mansard 8–12 weeks. Plan for around 4–7 months end to end — longer if you need full planning in a conservation area.
How to choose the right type — and keep it on budget
- Get your head height surveyed first — it rules types in or out before you spend on design.
- Get a proper Bill of Quantities so every builder prices the exact same scope — no apples-to-oranges quotes.
- Decide dormer vs mansard before you tender. Changing roof form mid-design is where loft budgets run away.
- Use a fixed-price contract, not day rates, and confirm what is in and out (staircase, en-suite, decoration, VAT).
- Serve party wall notices early so they do not delay your start.
Related guides
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Get a free estimateFrequently asked questions
- Which type of loft conversion is cheapest?
- A rooflight (Velux) conversion, from around £25,000–£45,000 in London, because it keeps the existing roof shape and adds no structure to the roofline. It only works if you already have enough head height.
- What is the difference between a dormer and a mansard?
- A dormer is a box built out from one part of the roof slope to create headroom. A mansard rebuilds the whole slope to near-vertical with a shallow top, adding most of a full storey — more space and cost, and it almost always needs planning permission.
- How much does a dormer loft conversion cost in London in 2026?
- Typically £45,000–£75,000 fully fitted for a rear dormer, depending on size, spec and whether you add an en-suite. It is the most common and usually best-value London loft conversion.
- Do I need planning permission for a loft conversion?
- Often not — rooflight and rear dormer conversions are frequently permitted development within 40 m³ (terrace) or 50 m³ (semi/detached). But front dormers, mansards, flats, listed buildings and homes in conservation areas or Article 4 zones need a planning application.
- What is the minimum ceiling height for a loft conversion?
- About 2.2 m over at least half the floor area is the practical working minimum. Below roughly 2.2 m at the ridge, a standard conversion becomes marginal, and a mansard or lowering the ceiling below is often the only route.
- Which loft conversion adds the most value?
- The one that adds a genuine double bedroom with an en-suite for the lowest cost your roof allows — usually a dormer on a terrace, or a hip-to-gable plus dormer on a semi. Value is capped by your street's ceiling price, so check that before choosing.